There seems to be a problem when two clients want to be connected to the tunnel.
The second client gets this error messages (on each tunnel connect):

remote tunnel on localhost:7869 is closed 12:13:40
Warning: remote port forwarding failed for localhost:7869 12:13:42

Is it impossible to debug with multiple clients on the same server domain, or is it a configuration problem?

Thanks!

It's because you can't open 2 listening sockets on the same port on the server (and after all it's what tunnel tried to do).
You have to assign unique port to each developer and don't forget to enable them in debugger.ports setting.